The limit is set by the slowing effect of interactions of the protons with the microwave background radiation over long distances (≈ 160 million light-years).
Due to the mean path associated with this interaction, extragalactic cosmic ray protons traveling over distances larger than 50 Mpc (163 Mly) and with energies greater than the threshold should never be observed on Earth.
[5] In this case, the argument behind the GZK limit does not apply in the originally simple form: however, as Greisen noted, the giant dipole resonance also occurs roughly in this energy range (at 10 EeV/nucleon) and similarly restricts very long-distance propagation.
A number of observations have been made by the largest cosmic-ray experiments Akeno Giant Air Shower Array (AGASA), High Resolution Fly's Eye Cosmic Ray Detector, the Pierre Auger Observatory and Telescope Array Project that appeared to show cosmic rays with energies above the GZK limit.
In the proposed Z-burst model, an ultra-high-energy cosmic neutrino collides with a relic anti-neutrino in our galaxy and annihilates to hadrons.
In July 2007, during the 30th International Cosmic Ray Conference in Mérida, Yucatán, México, the High Resolution Fly's Eye Experiment (HiRes) and the Pierre Auger Observatory (Auger) presented their results on ultra-high-energy cosmic rays (UHECR).
According to Alan Watson, former spokesperson for the Auger Collaboration, AGASA results have been shown to be incorrect, possibly due to the systematic shift in energy assignment.
After the flux suppression was established, a heated debate ensued whether cosmic rays that violate the GZK limit are protons.
The controversy was partially resolved in 2017, when a joint working group formed by members of both experiments presented a report at the 35th International Cosmic Ray Conference.
EUSO, which was scheduled to fly on the International Space Station (ISS) in 2009, was designed to use the atmospheric-fluorescence technique to monitor a huge area and boost the statistics of UHECRs considerably.
EUSO is to make a deep survey of UHECR-induced extensive air showers (EASs) from space, extending the measured energy spectrum well beyond the GZK cutoff.
Launched in June 2008, the Fermi Gamma-ray Space Telescope (formerly GLAST) will also provide data that will help resolve these inconsistencies.